1. Field of the Invention
The present invention pertains to water-activated batteries, i.e. deferred-action batteries which are activated when water or a similar liquid conductor comes into contact with the anode and the cathode inside the battery casing.
More specifically, the present invention relates to a water-activated immersion type battery for use on life vests, life rafts and in survival kits, which is capable of powering a signal light and able to withstand long storage at high and low temperatures and high humidity.
2. Description of the Related Art
The first water-activated batteries were developed in the 1940's by Bell Laboratories, and were further improved by General Electric Company. The early water-activated batteries were already characterized by a relatively high energy density. They were used primarily for military applications, such as in torpedoes and weather balloons, and for air-sea rescue equipment, pyrotechnic equipment, marine markers and emergency devices.
A seawater battery of the above-noted type is described, for example, in U.S. Pat. No. 3,966,497 to Harold Honer. The cells are disposed in a housing which is provided with two openings through which the electrolyte (seawater) flows into and out of the casing. The cells each comprise a sandwich formed of the anode, a sheet of flexible plastic material, and a cathode. As illustrated in the patent specification, the battery power lasts approximately three hours, with a reduction in the power output to below 80% after about 160 minutes.
Water-activated batteries, like many other types of batteries, create electric current by a chemical reaction between the anode and the cathode. When water enters the cell, it acts as a conductor permitting an ionic exchange between the anode and cathode to create an electric current. The anode, which may be made of magnesium, is progressively consumed during the reaction. The magnesium is transformed into magnesium hydroxide and magnesium chloride. The chloride is supplied by sodium chloride present in sea water and the hydrogen atoms are supplied by the water itself.
There are several types of water-activated batteries, including so-called immersion systems, and forced flow systems. Immersion type batteries are activated by immersion of the cell into a conducting liquid, solvent or electrolyte. Forced flow batteries are used as a power source for specialized applications, such as electric torpedoes. Sea water is forced through the battery as the torpedo travels through the water. Dunk type batteries are designed with absorbent separators between the electrolytes and are activated by pouring a conductive solution into the battery, where it is absorbed by the separators. The water-activated battery is constructed dry, stored in a dry condition and activated at the time of use by the addition of water or a conductive aqueous solvent.
Cathode materials have variously included silver chloride, copper thiocyanate-sulfur and magnesium dioxide. The most common cathode materials have been silver chloride and copper chloride. Also used are lead chloride, copper iodide-sulfur, copper thiocyanate-sulfur and manganese dioxide with a magnesium per-chlorate electrolyte.
Copper chloride became commercially available in 1949 and has lower energy densities, lower rate capability and less resistance to storage at high humidity, unlike silver chloride and copper chloride batteries. The latter are used primarily for airborne meteorological equipment, because silver chloride is more expensive and not warranted for most other applications.
The most widely accepted anode (negative plate) material is magnesium. Magnesium is the lightest structural metal in the industry. Magnesium is chemically active and thus is well suited for use in a water-activated battery. The anode is usually made from a type of magnesium alloy called AZ61 which is 6 percent aluminum, 1 percent zinc and 93 percent magnesium. AZ61 is preferably used because it deteriorates and polarizes less than other magnesium alloys. Other anode materials are used to produce various voltages. Zinc is selected for long life, but produces low power and low current in water-activated batteries. Zinc also produces minimal sludge. Aluminum is not a suitable anode because of its rapid formation of a protective, insulting oxide film (Al.sub.2 O.sub.3) on its surface.
Cathodes (positive plate) of conventional batteries are formed with a depolarizer and a current collector. The depolarizer takes the form of a powder which is electrically non-conductive. For the depolarizer to work, a form of carbon is added to impart conductivity. A chemical binder is also added to cause molecular attraction, and a metal grid is provided to collect the current. Silver chloride is a nonconductive cathode and is made conductive by reducing the surface to silver.
An example of a prior magnesium manganese-dioxide cell is illustrated in a paper entitled "Safe Annual Symposium Magnesium, The Safe Power Solution, Shiraz Dhanji, ACR Electronics, Inc." That type of battery, however, is not specifically designed for oceanic safety applications.